1811.) 
4 
of the calcareous genus, comprise all the 
various marbles and limestones. These 
are generally more free from admixture 
with other earths, than stones of the si- 
licious, or argillaceous kind; and their 
relative degrees of excellence for pur- 
poses of architecture are more easily 
ascertained by their external character. 
Bosides pure lime, they contain from 45 
to 50 per cent. of carbonic acid and 
water. Lime when pure is soluble in five 
hundred times its weight of water; and, 
even when united with carbonic acid, it 
is in a less certain degree soluble in river 
waters, owing to the minute portion of 
different acids, which is generally con- 
tained in them. On this account they 
are ill suited to form the foundations and 
piers of bridges, or to be employed in the 
construction of works exposed to the 
action of the water. The durability of 
marbles and lime-stones might; % be- 
lieve, with some certainty be determined 
by their relative degrees of hardness, and 
- by observing the time required to dissolve 
an equal cube of each kind of stone in 
marine acid, of the same strength, diluted 
with five times the quantity of water at 
the same temperature. The sediment 
remaining will also determine the quan- 
tity of silex. or clay with which the lime 
is combined. Magnesia enters into the 
composition of some lime-stones in the 
proportion of two-filths, and renders the 
softest stones of this kind less soluble in 
acids than the hardest marbles, on which 
account it will be necessary to ascertain 
_by chemical experiments, whether the 
slowness with which lime-stone is soluble, 
proceeds from'the presence of magnesia; 
but f£ believe it will also be found that a 
piixture of this earth, where it occurs in 
lime-stone, not only renders it less soluble 
in acids, but communicates to it a degree 
of durability which is not to be found in 
other lime-stones of the same degree of 
hardness, The high comparative degree 
of preservation observable in the exterior 
of York Minster, and other public edifices 
which are built of this stone, may serve 
to prove its excellence for purposes of 
architecture. 
Portland Stone is a peculiar kind of 
lime-stone, which some mineralogists 
cull roe-suine. When examined with.a 
magnifying lens, it will be found to con- 
tain «a number of small round globules, 
resembling in appearance the roes of 
fishes, imbedded in a calcareous basis, 
from whence it derives its name. It also 
coitains fragments of shells,.and minute 
éploargoup crystals, It varies in its quay 
tothe Selection of Stones for durable Architecture. 
405 
lities of hardness and compactness, and 
in its properties of durability, as may be 
observed in many of the public edifices 
in London, which are built of this stone. 
In the construction of St. Paul’s, some 
attention appears to have been paid to 
the selection of the stones for the ex- 
terior; which are more perfeet than thase 
in many buildings of a recent date; but 
they are evidently perishing in the upper 
part of this magnificent structure, 
Portland-stone contains carbonate of 
lime, united with a small portion of silex 
and clay. Its solution in diluted mu- 
riatic acid gives a dark-blue precipitate, 
with the Prussian alkali, indicating the 
presence of oxyd of iron, to which itowes 
its brownish tint; but the quantity of iron 
is tov small to affect its quality for the use 
of the architect. It burns to a white 
lime, losing more than eight parts in 
twenty of its weight, during calcination. 
According to Professor Jameson, roe- 
stone is never used for architecture, on 
account of its speedy disintegration; 
but his observations appear to have been 
confined to the varieties of this stone in 
Germany, and inapplicable to those in 
our own island. Two stones called by 
the same name, from different situations, 
are seldom exactly similar in all their 
properties ; which indeed rarely happens 
with stones from different layers of the. 
same bed, And where strata of calca- 
reous stone are separated by other 
kinds of stone, the upper and Jower 
strata, almost invariably differ in hard- 
ness and specific gravity; on which 
account it would be very desirable, that 
a mineralogical examination of stones 
should be made in their native quarries, 
and that those which ave intended for 
the external part of buildings, should be 
judiciously selected from the others. 
Of all stones of the calcareous genus, 
there cannot be a doubt that compact 
marbles, which can receive the highest 
degree of polish, would be the most 
beautiful and durable for the exterior of 
buildings; but their scarcity in this coun 
try prevents their application to this 
purpose.. Alabaster, which is composed 
of lime, united with sulphuric acid, from 
its beauty and the facility with which it 
can be worked, is used for ornamental 
architecture and sculpture; but the so- 
lubility of this stone renders it ill-suited 
to resist the agency of water, 
Dr. Watson relates, that he suspended 
two ounces of this stone in a pail of 
water for forty-eight hours, changing the 
water several times, and found ee i 
ai 
